The term “flip chip” denotes a method for interconnecting semiconductor devices, such as integrated circuit (IC) chips, to external circuitry with solder bumps that have been deposited onto the chip pads. IC dies having such solder bumps are referred to as “flip chips.” The solder bumps are typically deposited on the chip pads on the top side of a wafer during the final wafer processing step. In order to mount the chip to external circuitry, it is “flipped” over so that its top side faces down, and is positioned so that its pads align with matching pads on the external circuit. Then the solder in the solder bumps is flowed to complete the interconnect. After solder bonding is completed, a material known as underfill is applied to fill in the open spaces between solder bumps. The underfill is then heated to bond it to adjacent structure. The underfill provides added support to the bumps and helps to absorb stresses that might otherwise cause the solder bonds to fracture.
A flip chip may, in the manner discussed above, be attached directly to a printed circuit board. In some situations, the size and/or spacing of the flip chip solder bumps are different than those of the pads on the printed circuit board to which it is to be attached. In such situations, the flip chip may be provided in an integrated circuit (IC) package by mounting it on a chip carrier which is then directly attached to the printed circuit board. One type of chip carrier that is frequently used with a flip chip is known as ball grid array (BGA) carrier because it has a grid of solder balls on its bottom surface. An IC package having a flip chip operably mounted on the top surface of a ball grid array carrier is referred to as a flip chip ball grid array (FCBGA) package. In such a package the solder ball array on the carrier is usually considerably larger in scale than the solder bump grid on a flip chip, but each may contain the same number of contacts. An FCBGA package may be attached to a substrate in a manner analogous to that of flip chip attachment. The solder balls on the bottom of the FCBGA package are placed in contact with corresponding contact pads on the top surface of the substrate. The assembly is then heated, as in a reflow oven, to bond the solder balls to the contact pads. After solder bonding underfill material is applied between the solder balls and the assembly is reheated to bond the underfill to the package, substrate and adjacent portions of the solder balls.
Other types of flip chip packages include flip chip/pin grid array packages, column grid array packages, land grid array packages, and package on package packages. A flip chip may be connected to various types of electrical substrates (e.g., a printed circuit board, a chip carrier in an IC package, or other electrical substrate, all of which are referred to generically herein as “substrate”)
Electrical substrates may be made from different substances depending upon their intended use. Some electrical substrates are made from organic/plastic material and are known as organic or plastic substrates. Such substrates generally have copper pads in a pattern on the top surface of the substrate that matches the pattern of solder balls on a flip chip that is to be mounted on it. The flip chip is placed on the substrate, as with a pick and place machine, with solder bumps on the flip chip contacting corresponding pads on the substrate. The assembly is then heated, e.g., either in a reflow oven or by an infrared heater, causing the solder balls to liquefy or “flow.” Thereafter, the solder cools and solidifies, physically and electrically connecting the flip chip to the substrate.
One disadvantage associated with flip chips is that the solder bumps cannot flex in the way that longer leads can because the solder bumps are compact and not compliant. Bending during flip chip assembly and thermal cycling caused by a difference in coefficient of thermal expansion (CTE) between the organic substrate and the silicon flip chip can cause the organic substrate to warp. Such bending may also cause the underfill to delaminate from the substrate, which may lead to fractures in adjacent solder bonds.